The invention relates to a plasma spraying apparatus and also to a method for monitoring the condition of a plasma spraying apparatus.
Plasma spraying apparatuses, for example for coating the surface of a workpiece with a spray powder, are well known in the prior art, and are used widely in completely different technical fields. Known plasma spraying apparatuses often comprise a plasma spray gun, a high power direct-current source, a cooling aggregate and also a powder conveyer. For the protection of people and the environment, the spraying is often carried out in closed spaces which are equipped with vacuum filters, dust filters and noise protection devices.
In atmospheric plasma spraying an arc is triggered in a plasma torch between a water-cooled anode and a likewise water-cooled tungsten cathode. A process gas, usually argon or nitrogen or a mixture of an inert gas with nitrogen or hydrogen, is converted into the plasma state in the arc and a plasma beam with a temperature of up to 20.000 K develops. Particle speeds of 200 to 350 m/s are achieved through the thermal expansion of the gases. The powdery spray material enters the plasma beam with the help of a conveyor gas either axially or radially inside or outside of the anode region.
It goes without saying that in particular those components which are arranged close to the plasma torch can be affected in time by the extreme temperatures. The spray powder itself, which as a rule has aggressive and abrasive mechanical characteristics, also leads in time to wear on components of many different kinds, such as the powder injector, supply lines, seals, valves or the metering unit for the metering of the spray powder. Furthermore, a continuously constant quality of the spray powder is not always guaranteed. Thus it can, for example, happen that a powder charge contains particles which are much too large, so that clogging or narrowing of the supply cross-sections results at critical positions and the powder supply is reduced unduly or even interrupted completely. The possible operating faults which are only listed here by way of example, which occur during operation of a plasma spraying apparatus, all lead as a rule to the sprayed layers no longer meeting the required specifications so that the relevant workpieces have to be rejected in the worst case. Relatively minor damage or faults which can be remedied easily if recognized in time can, if they remain undetected, lead in the long run to further system components being damaged or rendered useless, which can lead to considerable service and repair costs that would not have occurred if the initially minor damage or faults had been discovered in time.